Setting environment variables from the build script via cargo::rustc-env

[chrjabs]

Hi all,

I have a crate with a C-API where I am using inline_c to test the C-API within the Rust test harness. In order for the tests to work, I need to set some flags for inline_c, which incline_c recommends to be done via cargo::rustc-env from the build script.
This method works with cargo test, however I noticed that it does not work with cargo-nextest. This is because inline_c builds the C tests at run time and therefore needs access to the environment variables set by the build script, but cargo-nextest does not set these environment variables.
My question now is, whether cargo-nextest reading the environment variables from the build script output and setting them is a reasonable feature, or whether inline_c is more or less abusing a cargo feature. The description of cargo::rustc-env just says "sets an environment variable", but the name suggests that these are intended to go to rustc rather than be used at runtime for tests.

I am aware that I could probably solve this with a setup script, but since setting the variables from the build script works for cargo test it might be worth considering supporting that as well.

As a proof of concept, with this (admittedly hacky for now) change, my tests work. If this sounds like a feature worth having, I would be up for implementing it more nicely.

diff --git a/nextest-runner/src/test_command.rs b/nextest-runner/src/test_command.rs
index 82955758..e3b6d90b 100644
--- a/nextest-runner/src/test_command.rs
+++ b/nextest-runner/src/test_command.rs
@@ -74,7 +74,22 @@ impl TestCommand {
         {
             // Convert the output directory to an absolute path.
             let out_dir = lctx.rust_build_meta.target_directory.join(out_dir);
-            cmd.env("OUT_DIR", out_dir);
+            cmd.env("OUT_DIR", out_dir.as_str());
+            // Set environment variables set via the `cargo::rustc-env` key
+            if let Ok(out_file) = std::fs::File::open(out_dir.parent().unwrap().join("output")) {
+                use std::io::BufRead;
+                for line in std::io::BufReader::new(out_file).lines() {
+                    let line = line.unwrap();
+                    let Some(key_val) = line
+                        .strip_prefix("cargo:rustc-env=")
+                        .or_else(|| line.strip_prefix("cargo::rustc-env="))
+                    else {
+                        continue;
+                    };
+                    let split = key_val.find('=').unwrap();
+                    cmd.env(&key_val[..split], &key_val[split + 1..]);
+                }
+            }
         }

         // Expose paths to non-test binaries at runtime so that relocated paths work.

[sunshowers]

Thanks for the question. I agree that nextest should be doing this (it takes great pains to run tests in the same environment as cargo), and I'm surprised this is the first time someone's run into this!

[chrjabs]

Right, I was also surprised to not find anything on this yet. I'll write up a cleaner implementation and send a PR then.